The present invention relates generally to the technology of transmission via optical fibers and more particularly to a system for improving the protection of data transmitted on a low-cost optical ring comprising only one optical fiber.
The use of optical fibers for new communication networks has become the norm. This applies in particular to networks known as federator or backbone networks installed by the major network operators, who have been deploying large quantities of these fibers for many years because of the intrinsic advantages associated with transporting data on optical fibers. This is due primarily to the enormous bandwidth that can be used to transport optical signals, which can be modulated at very high frequencies (expressed in gigabits (109 bits) per second) and which can nevertheless be propagated over long distances. The now generalized use of wavelength division multiplexing (WDM), enabling mixing in the same fiber of light at several different wavelengths, tens of different wavelengths or even hundreds of different wavelengths (in which case the expression dense WDM (DWDM) is used), has commensurately increased the transportation capacity of a single fiber and thus potentially of the network in which the fiber is deployed.
Nevertheless, the possibility of being able to transport enormous quantities of data on a permanent basis makes it essential to protect the data. All businesses, governments, banks and organizations of all kinds who are now using these networks on a permanent basis, and all their activities, including their most critical activities, depend on uninterrupted operation of the network, which means that the network must be able to recover its full functionality in a very short time period in the event of a fault or malfunction. This is the particular reason for the success of ring networks of the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) types, as respectively standardized in North America and Europe, which are for the most part compatible and standardize transmission speeds in the range indicated above, the fastest of which most widely used are 2.48 Gbit/s (SONET OC-48 or SDH STM-16), 10 Gbit/s (SONET OC-192 or SDH STM-64) and even 40 Gbit/s (SONET OC-768 or SDH STM-256). To ensure uninterrupted operation, this type of network comprises a double ring of optical fibers. One is a normally idle protection channel and is used only if the active fiber is broken or is affected by a major malfunction, after fast automatic protection switching (APS), necessitating less than 50 milliseconds, has been carried out to ensure traffic continuity, the essential quality criterion of networks of the above kind that transport enormous quantities of data.
Although techniques of the above type, i.e. WDM and transport protection based on a double ring of fibers, have been installed without difficulty in backbone networks, where their deployment cost is acceptable, the same cannot be said of other types of networks, such as metropolitan area networks, which are closer to the end user and whose deployment and operating costs cannot be shared between as large a number of users. Although the cost of plant for implementing the WDM technique has fallen sharply, opening the door to transporting multiple wavelengths on a single fiber, including in metropolitan area networks, and therefore opening the door to increasing their bandwidth, this generally does not go so far as providing a normally idle back-up fiber to be used only in the event of a fault. Nevertheless, the large number of users of a metropolitan area network, and the equally important activities that require access to the network, have just as great a requirement for protection as backbone networks.
This is why the object of the invention is to organize a ring network on the basis of a single fiber which, at the same time as allowing the use of new wavelength division multiplexing (WDM and DWDM) techniques to increase the bandwidth and functionality, nevertheless ensures a good level of protection of the data transported on networks in which deployment and operating costs are essential factors and in particular where it is not possible, or not feasible, to include an idle fiber used only in the event of a fault.
The invention therefore consists in a method of reconfiguring a ring optical network formed of a single optical fiber. The ring includes a traffic concentrator and stations optically connected to the fiber. The concentrator sends a light signal using a first group of wavelengths and receives a light signal using a second group of wavelengths, all the wavelengths used being different. The concentrator simultaneously sends to and receives from the stations via both ends of the fiber. When the network is established, a virtual break is created between two of the stations. On detection of a real break in the fiber, the virtual break is shifted so that it coincides with the real break, enabling traffic between the concentrator and the stations to be re-established.
To implement the invention, each station is equipped with a three-state optical switch forming a four-pole network transmitting light signals between its four poles in three operating modes, namely a direct mode, a crossed mode, and a transparent mode that is also the idle mode of the device.